I've got a Atomic 4 manifold with a stripped-out drain plug. I've found it difficult to tap the cast iron, so I went looking for some information on how best to create new threads in a hard iron casting. Along the way, I found some interesting web pages:

I also found this [http://www.ytmag.com/articles/artint89.htm Tinkers Guide to Manifold Repairs] which describes the types of cast iron and the ways to repair them, including welding, brazing and even epoxy repairs.